Motorhead Memo: Got time for a story?

By Kip Woodring

Once upon a time, 55 long years ago, a magazine of great repute tested a 1954 BSA Gold Star. Among many other irrelevant observations made during the test was one that is germane in the extreme—even today. See, they got this stock 350cc single-cylinder pushrod motorcycle to go well over 100 mph on low-octane fuel, with a measly 8.5:1 compression ratio. Now, I’ll grant you that the ultra-high gearing helped, but good lord, that’s fast for a small bore single a half century ago! (How fast will a Buell Blast go?) There’s a bit more to it than a stroll down memory lane, too.

You see, this was accomplished by running the engine up to 7,500 rpm (horrors!) using (wait for it)… a straight pipe! Putting the muffler back on the li’l thing cut some 800–900 rpm outta the deal and top speed dropped to about 92 mph—still faster than the 500cc Blast—in case you care. The moral of the story comes in the form of a situation that still exists with pushrod Harleys as we speak. (However, I predict it’s one that will not continue to exist for much longer.) Namely, that loud straight pipes do add a little top speed to engines that will rev higher with them installed. Still true, but the flip-side (equally true) is that the noise that accompanies such an exhaust swap is no longer tolerated. Frankly, it shouldn’t be… and wasn’t really even back in merry old England at the time of the Gold Star. That, in fact, was one reason for the building of the first racetrack in the world almost half a century before the aforementioned Gold Star road test! Brooklands, as it was called, was a high-speed test track that had muffler restrictions almost before it had speed trials. The so-called Brooklands muffler (Google it or read Buzz Buzzelli’s Sportster Performance Handbook—looks a lot like a fishtail) came into existence to comply with these noise rules of 90 years back, without eliminating any chance for the old-timers to go extremely fast on said race track. Basically, using a Brooklands with 10 times the volume of the engine meant going as fast as they could go with the straight pipe… yet quietly—geddit? You could say that the ol’ Brooklands can was the first (reasonably) successful attempt at a true performance exhaust, balancing speed and noise control with equal emphasis. And you would not be too far wrong.

But that was a lifetime ago. What’s changed since then? Sadly, not enough.

Well, since we all know that exhaust design requires a whole lot of thought about diameter, length, muffler volume and essentially the compromises required to make it effective at certain rpm ranges, it should come as no shock that in the 21st century most of ’em lag pretty badly. I’d say they are behind the times, since even the best of the modern designs tend to mimic the effects (if not the style) of that old Brooklands can. Seems to me the secret for future designs (such as are starting to show up in the car world already) is to build a system that’s optimal at all engine speeds and for all purposes. But how the hell do you do that? It seems the answer—ah—varies.

A hallmark of modern engines (even “old-fashioned” Harleys) is the onset of variable this, that and the other thing. I mean, we have variable ignition timing, variable valve timing, sometimes even variable cam timing, so it seems natural enough to me with all these modern electronic marvels in hand to apply it to an electronically variable exhaust system! Mind you—the look of the thing could take some getting used to but the last of the Buells had it, other sportbikes have it, Corvettes have it, even the old two-stroke Yamaha RZ350 had it in the primitive form of so-called YPVS, which effectively altered the height of the exhaust port to give those peaky little devils something resembling a power band! When you think about it, coupled with the design essentials we’ve looked at in the previous two segments of this “exhausting” discussion, it’s the only way forward. Otherwise, there are always going to be major limitations. As super-tuner D. William Denish put in his article entitled Exhaust Pipe Fundamentals— Exhausting Fundamentals:

“For optimum performance, the exhaust system must be matched to the engine’s induction system, cylinder heads and camshaft timing. These components should be tuned together as an integral system for maximum performance within a specific rpm range. If one component is changed or modified, the entire group of components must be retuned for maximum performance.

“An optimized exhaust system achieves a balance of pressure between the engine’s intake and exhaust tracts within a given rpm range. For a street engine, you want optimized torque in the low and midrange (2,500–4,500 rpm) for excellent acceleration and highway cruising along with decent power on the top end. However, every pipe design is a compromise. For example, if a pipe is designed for only bottom end torque, it will give up top-end horsepower and vice versa. Racers building large-displacement high-horsepower engines often design a pipe for top-end power to reduce low-end torque so the bike will launch easier, resulting in faster acceleration. An exhaust system is only efficient through a narrow range of the engine’s entire rpm band, so priorities must be set and compromises made to achieve the desired performance characteristics. Major exhaust system components include a mounting flange, head pipe, muffler and sometimes a merge collector. The diameter, length and overall design of these components will have a major impact on engine performance.”

Harley’s notion of “variable” thus far is restricted (literally!) to playing around with baffling and orifice sizes within the confines of traditional (smallish) H-D mufflers. Beyond those basics, the entire setup is intended mostly to vary the amount of flow (restriction) available to each individual head pipe. By cutting the flow (increasing restriction) through one muffler, the system forces more gasses through the other muffler via the crossover pipe in the header. Decent methodology for increasing mid-range power when you think about it, because opening everything up at high rpm only gives the best of both worlds… no restriction to peak power and superb mid-range, as well.

Less restrictive by far due to its huge size, the Buell version can do even better over an even broader rev-range, with the same technology—albeit a more sophisticated variation of that technology. Kids, you can try this at home if you want to, but it’s more than doubtful you can do a damn thing with it if you don’t have access to little things like super-computers and purpose-built flow test cells—damn complex stuff! Three-quarters of a century after bikers discovered certain basic pipe designs make motorcycles go faster, this is the next level of that thinking in this century.

Don’t think variable exhaust concepts are much more than in their infancy, either! Nor is Harley-Davidson in the forefront of the movement, either, as this U.S. patent for Japanese thinking on the subject (from 1990) shows. Here, we can see a system that’s capable of adjusting virtually every aspect of the exhaust function (as it relates to optimal engine function) with variable dimensions in head pipes as well as mufflers. Picture an ECM that interprets engine requirements, and incorporating variable valve-timing as well as ignition timing, load, throttle position, and more—then reads and varies the exhaust flow “configuration” into that which will perform best—in nano-seconds—continuously and constantly. To use an old expression from the days of compromised, primitive exhaust systems, there’s no better way to “keep it on the pipe.”

Variable exhaust dimensions, while perhaps not eliminating compromise, would sure minimize it! Imagine a system that could fool the engine into thinking it had the exact sizes, angles and lengths of pipe required for best overall performance at all times without excessive noise or restriction. Wow!

Thing is—it’s been tried before. The problem is really that it’s only been done (on Harleys) using modified versions of existing mufflers. That doesn’t necessarily cut it, sad to say.

On the other hand, the way Buell is (was) going about it, while not quite there in all ways yet, has fabulous potential. Erik’s so-called interactive exhaust system essentially means there are two paths through the muffler—long and short. A throttle- and ECM-actuated butterfly valve in the muffler controls the flow. At low rpm (and wide-open throttle) and high rpm, the valve is open to allow the engine to breathe faster and more easily. During the V-Twin’s mid-range, the valve is closed to increase back pressure, which amplifies acceleration torque. Slicker ’n hell and not excessively loud! It just flat works better overall than any other thing you can use on his motorcycles! Further, a similar system brought to us by Harley on our hogs would be just as good. Well, according to U.S. patent #7,347,045, as of March 25, 2008, Harley could provide us such a system! Currently, the technology is confined to a system oriented towards complying with noise laws while not killing power on (some CVO) stockers—but could there be a true performance system hidden in the bowels of a future Screamin’ Eagle catalog? Or similar concepts brought to market by far-sighted aftermarket companies? We’ll see…

Till then, let’s just leave it at this—there is no perfect 21st century performance exhaust for Harley-Davidsons! Everything out there owes more to that ancient Brooklands can and technology BSA Gold Star owners can relate to than any quantum leap into serious “future think” about pipes for hogs. Without being an exhaust engineer, you could be forgiven for thinking there’s gotta be a better way. The trick is to find it—or build it, if you can’t buy it.